1. Field of the Invention
The present invention relates to a zinc silicate system phosphor, and further to a method for producing the zinc silicate system phosphor, to a zinc silicate-system phosphor paste, and to a display device such as plasma display panel or the like comprising a phosphor layer including the zinc silicate system phosphor.
2. Description of Related Art
Recently, a plasma display panel (hereinafter, referred to as “PDP”) is capable of making a screen large and thin, and thereby, it attracts attention as a flat-panel display which can take the place of a cathode-ray tube (CRT). A PDP has two glass substrates comprising electrodes, and a number of micro discharge cells (hereinafter, referred to as “cells”) formed by partition walls provided between the substrates. In the inner wall of a partition wall, a phosphor layer is provided, and a discharge gas using Xe or the like as principal component is charged therein. The cells are arranged regularly on the substrate. When voltage is applied between the electrodes and the cells are discharged selectively, ultraviolet rays due to the discharge gas are generated, and thereby, the phosphors are excited and emit a visible light.
Presently, as phosphors mainly used for a PDP, there are (Y,Gd)BO3:Eu (Red), Zn2SiO4:Mn (Green), BaMgAl10O17:Eu (Blue) and the like. These phosphors are generally produced by a solid phase method, and their mean particle size is between 2 and 10 μm. The solid phase method is a method for obtaining a phosphor according to a solid phase reaction by mixing a predetermined amount of a compound including elements forming a ground material of the phosphor with a predetermined amount of a compound including activator elements such as Eu, Mn and the like, and calcining the mixture at a predetermined temperature.
Incidentally, with displays such as PDP and the like, improvement of brightness, smooth movie display and the like are required. Thereby, it is considered that improving the luminescence intensity of phosphors is effective in order to enhance the brightness. Particularly, a green phosphor has high visibility, so that it is important to enhance the luminescence intensity of the green phosphor in order to improve the white brightness. On the other hand, in order to achieve smooth movie display, information is required to be displayed in sequence per extremely short unit of time, so that phosphors having short persistence time are required.
The persistence time becomes short by increasing the amount of Mn in Zn2SiO4:Mn (Green). However, instead of that, the luminescent intensity deteriorates. Zn2SiO4:Mn has a structure such that a part of the Zn lattice points in the Zn2SiO4:Mn crystal is substituted for Mn, and the Mn becomes a luminescence center. However, the ionic radius of Mn2+ (0.080 nm) is larger than the ionic radius of Zn2+ (0.074 nm), and this causes lattice distortion or lattice defect in the crystal, which becomes a cause of deterioration of luminous efficiency.
Then, a method for decreasing lattice distortion or lattice defect caused in a crystal and improving the luminous efficiency by simultaneously substituting Mg having small ionic radius (the ionic radius of Mg2+: 0.066 nm) when substituting Zn lattice points for Mn has been known (See “Journal of electrochemical society” 146(6) 2353–2356 (1999); hereinafter, referred to as “Non-Patent Document 1”). Furthermore, since the ionic radius of Mg2+ is too small, the volume compensation effect of Mg2+ is supplemented and the luminous efficiency is further improved by substituting Ca2+ (the ionic radius: 0.099 nm) and Sr2+ (the ionic radius: 0.116 nm) together (See Japanese Patent Laid-Open Publication No. 2002-249767; hereinafter, referred to as “Patent Document 1”).
However, each phosphor in the above-described earlier technology (Patent Document 1 and Non-Patent Document 1) is produced by a solid phase method. No phosphors such that Mg2+, Ca2+ and the like are co-activated to Zn2SiO4:Mn as described above, according to a liquid phase method, and no methods of producing those phosphors is known.
Further, presently, along with miniaturization of the cell structure of a PDP, a phosphor having fine particles of not more than 1 μm is required. However, generally, with the solid phase method, in order to make fine particles of not more than 1 μm, it is required to perform crushing. Thereby, there is a possibility that lattice defect is generated in crystals according to stress applied at the time of crushing and the luminous efficiency deteriorates significantly. Furthermore, since the solid phase method is for producing phosphors according to solid-phase reaction, there remain nonreactive excessive impurities, excess soluble salts generated by reaction, and the like. Therefore, it is difficult to obtain a phosphor having high purity, and to control the content of activators in the ground material of crystals.